Quarterly report pursuant to Section 13 or 15(d)

NONCONTROLLING INTERESTS - Narrative (Details)

v3.10.0.1
NONCONTROLLING INTERESTS - Narrative (Details) - $ / shares
6 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Class of Stock [Line Items]    
Series A-1 preferred units stated value (in dollars per share) $ 0.01 $ 0.01
Common shares of beneficial interest, par value (in dollars per share) $ 0.01 $ 0.01
Series A-1 Preferred Units    
Class of Stock [Line Items]    
Series A-1 preferred units dividend rate (percent) 6.00%  
Series A-1 preferred units stated value (in dollars per share) $ 25  
Series A-1 Preferred Units | NSA OP, LP    
Class of Stock [Line Items]    
Unit conversion ratio 1  
OP units | NSA OP, LP    
Class of Stock [Line Items]    
Unit conversion ratio 1  
Minimum conversion period (in years) 1 year  
Issuance of shares/units (in shares) 2,024,170  
Redemptions/conversions of units (in shares) 246,582  
OP units | NSA OP, LP | Self Storage Properties    
Class of Stock [Line Items]    
Issuance of shares/units (in shares) 464,263  
OP units | DownREIT Partnership    
Class of Stock [Line Items]    
Unit conversion ratio 1  
Minimum conversion period (in years) 5 years  
Subordinated performance units    
Class of Stock [Line Items]    
Percentage of cash available for distribution used in conversion ratio calculation 110.00%  
Subordinated performance units | NSA OP, LP    
Class of Stock [Line Items]    
Redemptions/conversions of units (in shares) 997,074  
Subordinated performance units | NSA OP, LP | Self Storage Properties    
Class of Stock [Line Items]    
Issuance of shares/units (in shares) 56,228  
Subordinated performance units | DownREIT Partnership    
Class of Stock [Line Items]    
Unit conversion ratio 1  
Minimum conversion period (in years) 5 years  
Subordinated performance units | NSA OP, LP And DownREIT Partnership    
Class of Stock [Line Items]    
Unit conversion ratio 1  
Minimum conversion period (in years) 2 years  
Unit conversion, lock out period 2 years  
LTIP units | NSA OP, LP    
Class of Stock [Line Items]    
Unit conversion ratio 1